Compute constants

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+/*
+	This file is part of cpp-ethereum.
+
+	cpp-ethereum is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	cpp-ethereum is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with cpp-ethereum.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/** @file ConstantOptimiser.cpp
+ * @author Christian <c@ethdev.com>
+ * @date 2015
+ */
+
+#pragma once
+
+#include <vector>
+#include <libdevcore/CommonData.h>
+#include <libdevcore/CommonIO.h>
+
+namespace dev
+{
+namespace eth
+{
+
+class AssemblyItem;
+using AssemblyItems = std::vector<AssemblyItem>;
+class Assembly;
+
+/**
+ * Abstract base class for one way to change how constants are represented in the code.
+ */
+class ConstantOptimisationMethod
+{
+public:
+	/// Tries to optimised how constants are represented in the source code and modifies
+	/// @a _assembly and its @a _items.
+	/// @returns zero if no optimisations could be performed.
+	static unsigned optimiseConstants(
+		bool _isCreation,
+		size_t _runs,
+		Assembly& _assembly,
+		AssemblyItems& _items
+	);
+
+	struct Params
+	{
+		bool isCreation; ///< Whether this is called during contract creation or runtime.
+		size_t runs; ///< Estimated number of calls per opcode oven the lifetime of the contract.
+		size_t multiplicity; ///< Number of times the constant appears in the code.
+	};
+
+	explicit ConstantOptimisationMethod(Params const& _params, u256 const& _value):
+		m_params(_params), m_value(_value) {}
+	virtual bigint gasNeeded() = 0;
+	virtual void execute(Assembly& _assembly, AssemblyItems& _items) = 0;
+
+protected:
+	size_t dataSize() const { return std::max<size_t>(1, dev::bytesRequired(m_value)); }
+
+	/// @returns the run gas for the given items ignoring special gas costs
+	static bigint simpleRunGas(AssemblyItems const& _items);
+	/// @returns the gas needed to store the given data literally
+	bigint dataGas(bytes const& _data) const;
+	/// @returns the gas needed to store the value literally
+	bigint dataGas() const { return dataGas(toCompactBigEndian(m_value, 1)); }
+	static size_t bytesRequired(AssemblyItems const& _items);
+	/// @returns the combined estimated gas usage taking @a m_params into account.
+	bigint combineGas(
+		bigint const& _runGas,
+		bigint const& _repeatedDataGas,
+		bigint const& _uniqueDataGas
+	)
+	{
+		// _runGas is not multiplied by _multiplicity because the runs are "per opcode"
+		return m_params.runs * _runGas + m_params.multiplicity * _repeatedDataGas + _uniqueDataGas;
+	}
+
+	/// Replaces the constant by the code given in @a _replacement.
+	void replaceConstants(AssemblyItems& _items, AssemblyItems const& _replacement) const;
+
+	Params m_params;
+	u256 const& m_value;
+};
+
+/**
+ * Optimisation method that pushes the constant to the stack literally. This is the default method,
+ * i.e. executing it does not alter the Assembly.
+ */
+class LiteralMethod: public ConstantOptimisationMethod
+{
+public:
+	explicit LiteralMethod(Params const& _params, u256 const& _value):
+		ConstantOptimisationMethod(_params, _value) {}
+	virtual bigint gasNeeded() override;
+	virtual void execute(Assembly&, AssemblyItems&) override {}
+};
+
+/**
+ * Method that stores the data in the .data section of the code and copies it to the stack.
+ */
+class CodeCopyMethod: public ConstantOptimisationMethod
+{
+public:
+	explicit CodeCopyMethod(Params const& _params, u256 const& _value);
+	virtual bigint gasNeeded() override;
+	virtual void execute(Assembly& _assembly, AssemblyItems& _items) override;
+
+protected:
+	AssemblyItems m_copyRoutine;
+};
+
+/**
+ * Method that tries to compute the constant.
+ */
+class ComputeMethod: public ConstantOptimisationMethod
+{
+public:
+	explicit ComputeMethod(Params const& _params, u256 const& _value):
+		ConstantOptimisationMethod(_params, _value)
+	{
+		m_routine = findRepresentation(m_value);
+	}
+
+	virtual bigint gasNeeded() override { return gasNeeded(m_routine); }
+	virtual void execute(Assembly&, AssemblyItems& _items) override
+	{
+		replaceConstants(_items, m_routine);
+	}
+
+protected:
+	/// Tries to recursively find a way to compute @a _value.
+	AssemblyItems findRepresentation(u256 const& _value);
+	bigint gasNeeded(AssemblyItems const& _routine);
+
+	AssemblyItems m_routine;
+};
+
+}
+}